Date: Wed, 3 Jul 2013 02:02:10 -0500 From: Kevin Day <toasty@dragondata.com> To: Will Andrews <will@firepipe.net> Cc: freebsd-fs <freebsd-fs@freebsd.org> Subject: Re: EBS snapshot backups from a FreeBSD zfs file system: zpool freeze? Message-ID: <A5A66641-5EF9-454E-A767-009480EE404E@dragondata.com> In-Reply-To: <CADBaqmihCB5JP01hLwXTWHoZiJJ5-jkT-Ro=oDwOcKZT_zvEKA@mail.gmail.com> References: <87li5o5tz2.wl%berend@pobox.com> <CA%2BtpaK1jQuKneQsxkVfxJGzXdPdLZfqBM1QWQ0e19nK5t71t1Q@mail.gmail.com> <87ehbg5raq.wl%berend@pobox.com> <20130703055047.GA54853@icarus.home.lan> <6488DECC-2455-4E92-B432-C39490D18484@dragondata.com> <CADBaqmihCB5JP01hLwXTWHoZiJJ5-jkT-Ro=oDwOcKZT_zvEKA@mail.gmail.com>
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--Apple-Mail=_900D5931-C93C-4EBD-AEE4-DD9657BD91AF Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=iso-8859-1 On Jul 3, 2013, at 1:53 AM, Will Andrews <will@firepipe.net> wrote: > On Wednesday, July 3, 2013, Kevin Day wrote: > The closest thing we can do in FreeBSD is to unmount the filesystem, = take the snapshot, and remount. This has the side effect of closing all = open files, so it's not really an alternative. >=20 > The other option is to not freeze the filesystem before taking the = snapshot, but again you risk leaving things in an inconsistent state, = and/or the last few writes you think you made didn't actually get = committed to disk yet. For automated systems that create then clone = filesystems for new VMs, this can be a big problem. At best, you're = going to get a warning that the filesystem wasn't cleanly unmounted. >=20 > Actually, sync(2)/sync(8) will do the job on ZFS. It won't stop/pause = I/O running in other contexts, but it does guarantee that any commands = you ran and completed prior to calling sync will make it to disk in ZFS. >=20 > This is because sync in ZFS is implemented as a ZIL commit, so = transactions that haven't yet made it to disk via the normal syncing = context will at least be committed via their ZIL blocks. Which can then = be replayed when the pool is imported later, in this case from the EBS = snapshots. >=20 > And since the entire tree from the =FCberblock down in ZFS is COW, you = can't get an inconsistent pool simply by doing a virtual disk snapshot, = regardless of how that is implemented. >=20 > --Will. Sorry, yes, this is true. We're not using ZFS to clone and provision new = VMs, so I was just thinking about UFS here. And ZFS does have a good = advantage here that it seems to actually respect sync requests. I think = it was here I reported a few months ago that we were seeing UFS+SUJ not = actually doing anything when sync(8) was called. But for some workloads this still isn't sufficient if you have processes = running that could be writing at any time. As an example, we have a = database server using ZFS backed storage. Short of shutting down the = server, there's no way to guarantee it won't try to write even if we = lock all tables, disconnect all clients, etc. mysql has all sorts of = things done on timers that occur lazily in the future, including = periodic checkpoint writes even if there is no activity. I know this is a sort of obscure use case, but Linux and Windows both = have this functionality that VMWare will use if present (and the guest = tools know about it). Linux goes a step further and ensures that it's = not in the middle of writing anything to swap during the quiesce period, = too. I don't think this would be terribly difficult to implement, a hook = somewhere along the write chain that blocks (or queues up) anything = trying to write until the unfreeze comes along, but I'm guessing there = are all sorts of deadlock opportunities here. Either way, I'm not asking that anyone spend time to write this, I'm = just trying to reword what the original requestor was talking about. -- Kevin --Apple-Mail=_900D5931-C93C-4EBD-AEE4-DD9657BD91AF Content-Disposition: attachment; filename=smime.p7s Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIIPLzCCBN0w ggPFoAMCAQICEHGS++YZX6xNEoV0cTSiGKcwDQYJKoZIhvcNAQEFBQAwezELMAkGA1UEBhMCR0Ix GzAZBgNVBAgMEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBwwHU2FsZm9yZDEaMBgGA1UECgwR Q29tb2RvIENBIExpbWl0ZWQxITAfBgNVBAMMGEFBQSBDZXJ0aWZpY2F0ZSBTZXJ2aWNlczAeFw0w NDAxMDEwMDAwMDBaFw0yODEyMzEyMzU5NTlaMIGuMQswCQYDVQQGEwJVUzELMAkGA1UECBMCVVQx FzAVBgNVBAcTDlNhbHQgTGFrZSBDaXR5MR4wHAYDVQQKExVUaGUgVVNFUlRSVVNUIE5ldHdvcmsx ITAfBgNVBAsTGGh0dHA6Ly93d3cudXNlcnRydXN0LmNvbTE2MDQGA1UEAxMtVVROLVVTRVJGaXJz dC1DbGllbnQgQXV0aGVudGljYXRpb24gYW5kIEVtYWlsMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8A 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